This invention relates to digital demodulation arrangements for quadrature signals formed in a zero I.F. or direct conversion radio receiver.
In British patent application No. 8127797 (Ser. No. 2106734A) there is described a direct conversion receiver incorporating a multi-mode digital demodulator, using the configuration shown in FIG. 1. In this receiver, the input signals from the antenna (or from the cable system or whatever) are mixed in two mixers 1, 2 with a local oscillator 3. In either the oscillator or signal path a quadrature network is interposed 4 or 4' such that the relative phases of either the signal or the local oscillator have a phase difference of 90.degree. at the two mixers. (As an alternative, two 45.degree. phase shift networks of opposite sign may be used, one in the signal and one in the local oscillator). The outputs from the mixers are low-pass filtered 5,6 to select the difference frequency between the input signal and the local oscillator. After amplification 7,8 (if necessary) the signals in the two channels (termed the `I` or in-phase and the `Q` or quadrature channels) are converted from analogue to digital form I', Q' by elements 9, 10. The digital signals are processed in a digital signal processing block 11, which may consist of hardwired logic or which may be a microprocessor(s) programmed with software. The processor performs the demodulation of the signals and may also provide filtering and other post-detection functions. It further may provide specialised outputs such as feedback to the other receiver parts to control gain or phase--such feedback being either digital or converted to analogue via a digital to analogue converter.